Casing centering device



Jan. 12, 1954 w. s. ALTHOUSE, JR 2,665,762

CASING CENTERING DEVICE Filed Aug. 28, 1951 5 Sheet sSheet 2 i a/z'y, 5',

Casing :9

Casing :5

Wu. IAM 5. AL n/ousz, r12.

IN V EN TOR.

Arrow/v5 Y$ 5 Sheets-Sheet 5 Filed Aug. 28, 1951 EEG $87 s/v/adg 215 5.9321

mil/AM S A1. 7/1 0055, J2.

IN VEN TOR.

ATTae/vEYS Patented Jan. 12, 1954 CASING CENTERING DEVICE William S. Althouse, Jr., Arcadia, Calif., assignor to Baker Oil Tools, Inc., Vernon, Calif., a corporation of California Application August 28, 1951, Serial N 0. 243,975

3 Claims.

The present invention relates to devices for centering well casing, liners and similar conduits in bore holes.

Centering devices are mounted on strings of well casing, and similar well conduits, to hold the latter spaced away from the walls of holes in which they are disposed, usually for the purpose of insuring the deposition of a uniform annulus of cementitious material around the well casing, in order to obtain a satisfactory cement job. One type of centering device now being employed for the above purpose embodies outwardly bowed spring members that bear against the wall of the hole, tending to press and hold the casing away from the wall. If the well casing is to be disposed in a hole that is much greater in diameter than the casing, then the springs should have a large bowed height, if they are to engage the wall of the hole and exert a restoring force tending to center the casing in the hole. Similarly, despite the fact that the hole diameter may not be very much greater than the casing diameter, the centralizer may land opposite a large cavity in the wall of the hole and be incapable of holding the casing spaced from the wall of the hole, unless springs with large bows are used.

The use of springs with large bowed heights in casing centering devices has several disadvantages associated with it. The bowed portion of a spring can be pressed inwardly closer to the casing on which the centering device is mounted only through the application of progressively greater forces against it; so that with a large bowed height, a comparatively large compressive force has heretofore been required to deflect the spring bow close to the casing. In tarting the centering device mounted on a casing string into close clearance surface casing, the large height bowed portions must be compressed inwardly to a great extent, since the surface casing usually is not very much greater in diameter than the casing string itself. As a result, the bowed portions of the springs must be subjected to comparatively large forces in compressing them through the greater portion of their deflection toward the casing string, which requires a corresponding large force or load to be imposed downwardly upon the casing string. In the usua1 situation, with springs having large bowed heights, the centering device cannot be started in the close clearance surface casing merely by the weight of the casing itself, on which the centering device is mounted, but resort must be had to the application of external weight or forces on'the casing string. That is, the casing string must be snubbed into the surface casing.

In addition to a large force being required to start the centering device in the surface casing, the comparatively great force being exerted by the springs on the wall of the close clearance surface casing creates correspondingly great sliding friction between the springs and the surface casing, which must be overcome in lowering the casing string on which the centering device is mounted, and which may create wear on the springs to the extent that they are incapable of later centering the casing string in the hole,

Centering springs with large bowed heights have the further disadvantage of possibly becoming permanently distorted; that is, taking a permanent set when deflected to a large extent close to the casing string on which the centering device is mounted.

Accordingly, an object of the present invention is to provide a casing centering device embodying springs with comparatively large bowed heights that require less force to start them in the surface casing, exert less sliding friction against the wall of the surface casing, or other relatively great restriction through which the device must pass, but which, nevertheless, exert a comparatively high restoring force tending to center the casing, when engaging the wall of the hole at large distances from the exterior of the casing on which the device is mounted.

Another object of the invention is to provide a casing centering device embodying springs with comparatively large bowed heights, in which the springs are effective to exert adequate force, tending to restore the casing toward the center of the hole, when the spring bows are at large distances from the casing on which the device is mounted, and in which the spring force is of a relatively low value when its bowed portion is pressed close to the casing on which the device is mounted.

A further object of the invention is to provide a casing centering device embodying outwardly bowed springs, in which the restoring force exerted by the springs and tending to center the casing on which the device is mounted is of a sufficient value in the normal working range of the springs, that is, when the spring bows engage the wall of the hole at normal heights from the casing, and in which the restoring force exerted by the springs is of a comparatively low value when the casing is moved closer to the wall of the hole containing the casing. By virtue of such a device, springs with large bowed heightscan perform their purpose of centerin the cas ng in large holes, and when disposed in hole cavities, without the springs taking a permanent set when moved into and through relatively small restrictions, such as surface casing having relatively small clearance for the casing on which the centering device is mounted.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown in the drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illustrating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Referring to the drawings:

Figure l is a side elevation of a casing centering device mounted on a string of casing;

Fig. 2 is a longitudinal section, partly in elevation, of the centering device mounted on a string of casing and disposed in a bore hole;

Fig. 3 is a cross-section taken along the line 3--% on Fig. 1;

Fig. 4 is an enlarged side elevation of one of the outwardly bowed springs embodied in the centering device;

Fig. 5 is a view similar to Fig. 4 of a prior art form of outwardly bowed spring;

Fig. 6 is a graph representing the restoring forces at various distances the casing is held from the wall of the hole exerted by complete centerin devices having different shaped springs when mounted on the casing;

Fig. 7 is a graph similar to Fig. 5 comparing the restoring forces of centering devices embodying different shaped springs, including the specific forms illustrated in Figs. 4 and 5;

Fig. 8 is a graph illustrating the starting and sliding forces of different forms of springs when run in surface casings having diiferent diametral clearance with the casing on which each centering device is mounted.

The invention is disclosed in the drawings in connection with a specific design of casing centering device merely for the purpose of illustration. The invention is applicable to centering devices having outwardly bowed springs in general, regardless of the specific construction of the portions of the centering device other than the outwardly bowed springs themselves.

A casing centralizer A is mounted upon a casing string, which includes an upper casing section B, a lower casing section C, and an intervening coupling collar D threadedly secured to the opposed pin. ends l0, iii of the casing sections, in order to hold them rigidly together. In the specific form of the invention illustrated, the coupling collar D functions as a stop member, tending to pull the casing centralizer A through the well bore, regardless of the longitudinal direction of movement of the casing string therein.

The casing centralizer includes an upper cage assembly ii, that can be mounted on the upper casing section B above the coupling collar D, and a lower cage assembly i2, that can be mounted on the lower casingsection 0 below the coupling collar D. These cage assemblies H, l2 are secured to one another by intervening outwardly bowed leaf springs l3, which extend completely across the coupling collar or stop member D.

Each cage assembly i I or i2 includes an outer spring annulus or collar M and an inner stop annulus or collar [5, these two collars being connected together by a rigid interconnecting member or bar l6, whose inner and outer ends abut and are welded to the inside surfaces of the collars. The interconnecting bars it; preferably do not extend laterally beyond the inner or outer surfaces of the collars l5, l5. Suflicient bars are provided, circumferentially spaced from one another, to form a comparatively rigid cage assembly I and I 2.

The outwardly bowed springs l3 are preferably uniformly spaced around the cage members II, I2 and have their end portions isa secured to the spring collars M. As an example, the ends |3a may be disposed in inner notches i! in the collars l4, and they may be welded to the spring collars, as by the aid of welding material l8 extending across their outer ends.

For the purpose of enabling the casing centralizer A to be mounted upon an assembled casing string, which already possesses a stop member, such as the coupling collar D, each cage member is made sectionaL'with its spring collar [4 and stop collar l5 composed of two main or substantially semi-circular pieces. These pieces can be secured together by hinges l9, 2!] to form a circumferentially continuous spring collar and stop collar. As disclosed in the drawings, at one side of the cage the two parts of the spring collar M are securedto one another by a hinge l9. The leaves 25 of the hinge are spot welded, or otherwise suitably secured, to the outer surface of the collar sections, whereas the knuckle or barrel portions 22 of the hinge are interleaved with respect to one another, and are disposed between the adjacent ends of the collar sections. A suitable hinge pin 23 extends through the knuckles 22, to pivotally secure the collar sections to one another.

A similar hinge arrangement is provided on the same side of the cage for the stop collar 25. The hinge arrangements are provided on both the upper and lower cages, the hinges Hi all being in alignment with one another; so as to enable the centralizer to be swung between open and closed positions about the hinge axis of the pins 23 in mounting the centralizer on the casing string, with the upper and lower cages ll, i2 on opposite sides of the coupling collar D.

The other side of the cages ll, i2 are similarly provided with hinges Ed in alignment with one another, the hinge leaves 25 being spot welded, or otherwise suitably secured, to the exteriors of the collars l4, i5 and the knuckles or barrels 22 being disposed between the adjacent ends of the collar sections.

The hinge pins 23 at one side of the casing centralizer are in place and form the pivotal axis of swinging the casing centralize! between open and closed positions. The other hinge members 25 originally do not have a hinge pin extending through their interleaved barrel portions 22. After the centralizer has been swung to open position about the hinge axis on one side containing the hinge pins 23, and has been placed around the casing string, it can be swung to closed position with the barrels 22 of the hinges 2d interleaved with respect to one another. A pin 25 may then be inserted within the knuckles 22 of each hinge 2i}, for the purpose of holding the collars M, l5 of the cages H, 52 snugly around the casing sections B, C in slidable relation with respect thereto.

After the casing centralizer A has been mounted on the casing string, the combination is lowcred through the well bore. During this lowering movement, the coupling collar D engages the lower stop collar I15, the downward movement or force being transmitted through the rigid bars or columns it to the lower spring collar 1'4 (see 2). Since the lower ends I30. of the sprin s are firmly secured to the lower spring collar M, the springs iii will be pulled through the well bore, and through any restrictions that might be encountered therein. When a. restriction or tight place is encountered, the springs are urged inwardly toward the casingstring, which shifts the upper cage ll relatively in an upward direction away from the coupling collar D.

In a similar manner, upward movement of the casing string will cause the coupling member D to engage the upper stop collar it, in order to exert a pulling action on the outwardly bowed spring members [3, which facilitates their passage through a well bore and through any restrictions therein that might be encountered.

As disclosed in the drawings, each outwardly bowed spring i3 is a one-piece unit, consisting of several specific portions (see 4). The central or middle portion 3t of the spring curved in convex fashion facing in a laterally outward direction from the axis of the centering device. Each end portion iii of the spring is curved in a concave fashion facing laterally outward of the axis of the casing. centering device. Each concavely curved portion 3! preferably terminates in a relatively fiat end 32, to insure the proper relationship and welding of such end to the adjacent spring collar M and to insure that the spring will flex or bend upon the casing sections B orC at apoint that is removed from the weld i Between the convex central portion 332 and each end portion 31, the spring is constituted by a "lat interconnecting portion 33 of substantial length. Thus, the convex portion ges smoothly into the flat portion 33 at the po of tangency 3d of the flat portion with the central portion. Each concave portion 3! also merges smoothly into the flat portion 33 at the point or" tangency of the fiat portion 33 with respect to the concave portion 3!. Although a straight or fiat intermediate portion 33 is preferred, interconnecting the convex and concave portions of each spring, the objectives of the present invention can also be efiectuated by making such inter mediate portion substantially straight or flat, as by forming it as a curve having a comparatively large radius of curvature when compared to the respective radii of curvature of the convex and concave portions 35!, 3i of the spring.

The provision of the intermediate flat portion 33 between the central convex portion 39 and each end portion 3! of the spring enables :a con tering device A to be provided with springs 13 having a comparatively large bowed height 35.; so as to be capable of centering the casing S in relatively large holes. The centering device has adequate restoring force tending to press the casing string S away from the wall of the hole 1- over the working range of the device. Despite this fact, actual tests have demonstrated that the springs 13 of the centering devices are not subjected to excessive forces when compressed inwardly; so as to place the central bowed p01?" tions 3% close to the casing 55. These tests show that the amount of force necessary to press the bowed. portion 36 of the spring toward the casing does not increase in direct proportion to the distance that the convex bow is moved toward the casing, but that the force required increases to a much lesser extent. In other words, the restoring or reactive force exerted by each spring increases comparatively little as it is moved toward the casing S on which the centering device is mounted, when compared to spring arrangements that do not have the intermediate sub stantially flat portions 33 between the central convex portion 3 8 and the end concave portions 3!.

Because of the .fact that the restoring force exerted by the spring [3 does not increase greatly it is compressed toward the casing, the centering device can be started in close clearance surlace casing in a comparatively easy manner by subjecting the springs to the axial weight of the casing its-elf on which the centering device is mounted. It is unnecessary to supplement the weight of this casing by any external forces, as by snubbing the casing, in order to start the centering cevice into the surface casing. Once having been inserted in the surface casing, applicants spring arrangement reduces the amount of sliding force exerted by the springs considerably, over prior-devices.

The efiicacy of the intermediate, substantially flat portions 33 can be demonstrated by reference to the various curves shown in the drawings, which indicate the forces exerted by outwardly bowed springs of dilferent specific design, when their bowed portions are compressed to different distances from the casing on which the centering device is mounted. Fig. 6 constitutes various curves plotting restoring force, as an ordinate, against the distance the casing is held oil the wall of the confining hole, as an abscissa. Curves F, G, H, K, L, M, N, P are each derived from. outwardly bowed spring centralizers, whose springs have the same unrestrained bowed height, which was actually a six-inch rise in each case. The over-all length of each spring varied somewhat, which was also true of the radius of its convex central portion 30, the radius of the concave end portions 3|, and the extent of the intermediate fiat portion 33, if any. Each spring was made from the same material, had the same cross-section, and was subjected to the same heat treatment.

The following table gives the dimensional and physical data on the springs used in the centering devices from which the curves shown in Fig. 6 were obtained:

Dimensional and physical data on springs i Center End (0011- lntersplmgtpeslgna' (Convex) cave) mediate Radius Radius Flat 15% I 31% I None 48 10 f 35 None 46 30%ui 15 None 30 1 8 None 41 64 8 8 u 41. /64 8 I 8 l 13% 3T %4 8 l 8 12 %4 35 10 I 10 f 10% 35 344 From the foregoing table, it is evident that springs F, G, H and K did not have any intermediate flat portion between the central convex portion 3!! and each end concave portion 3 i that is, the central portion merges smoothly into each greatly increasing restoring force as the casing on which each particular centering device is mounted is moved closer to the wall of the hole. Each of these curves rises with accelerated or increasing steepness as the spring is compressed closer to the casing. Accordingly, centering devices made with springs have an ogive curve, in which the convex central 3H) and concave end portions 3! meet at a common tangent or flex point, are diffcult to start in close clearance surface casing, in View of the comparatively great force required to compress them close to the casing on which the centering device is mounted. This force may be of such an extent as to cause the springs to take a permanent set; so that they may not reassume their initial outwardly bowed form when disposed in open hole below the surface casing, in order to urge the casing away from the wall of the hole. In addition, such great force exerted by the springs in close clearance surface casing also introduces comparatively large sliding friction forces between the springs and the wall of the casing, causing wear on the springs.

As compared with the steeper rise of the curves for the springs F, G, H, and K as the casing is moved closer to the wall of the hole, springs L, M, N and P, which have the intermediate flat portion 33 between the central portion 33 and each end concave portion 3 i, exert a substantially lesser restoring force as their bowed portions are compressed toward the casing, when the casing is moved off-center toward the wall of the hole. Springs L, M, and N each have the central convex portion 3%! with the same radius of curvature as the concave end portions 3i, as seen in the above table. However, spring L has a longer intermediate flat portion than spring M, which, in turn, has a longer intermediate fiat portion than spring N. Spring L, with the longest intermediate flat portion 33, exerts an adequate restoring force when held four inches off the wall of the hole, as seen in the curve drawn on Fig. 6, but this restoring force does not increase very greatly as the casing is moved laterally toward the wall of the confining hole. As a matter of fact, curve L shows that instead of the restoring force increasing by accelerated amounts as the casing is moved closer to the wall of the hole, the restoring force actually increases by lesser or decelerated amounts, since the curve L tends to flatten oii. Curve M for spring M, which has the shorter intermediate flat or straight spring portion 33, exerts a greater restoring force as the distance the casing is held off the wall decreases than the curve L for spring L; but here again, the restoring force tends to flatten off as the casing is moved laterally toward the wall of the hole, increasing by decelerated amounts, rather than by accelerated amounts. The same is true of curve N for spring N that has a still lesser intermediate flat portion 33 between the central and end curved portions 3t, 3! of the spring. The restoring force is greater throughout, but the curve also flattens off as the casing is moved closer to the wall of the hole, to shift the bowed portions of the spring closer to the casing on which the centering device is mounted.

Curve P for spring P has a greater radius of curvature for the central and end portions 3!), 31 of the springs, but has a smaller length of the intermediate or strai ht flat portions 33. This spring P, as exemplified by curve P, also tends to flatten off as the casing is moved laterally closer to the wall of the hole, the centering device exerting an adequate restoring force when the casing is held four inches off the wall of the hole, this restoring force being not excessive when the casing is disposed only one inch off the wall of the hole. If the spring P did not have the intermediate substantially flat portions 33, then the curve P would have risen very steeply as the casing was moved closer to the wall of the hole, the springs exerting an excessive restoring force, which is unnecessary for the purpose of holding the casing away from the wall of the hole, but which would make it difficult to start the centering device in closed clearance surface casing, and greatly increase the slidable frictional forces between the springs and the wall of the surface casing, and might even cause the springs to take a permanent set'or inwardly collapsed deformation.

From a comparison between curves F, G, H and K, on the one hand, and curves L, M, N and P, on the other hand, it is apparent that the intermediate flat portion 33 between the central convex curve portion 3 and end concave portions 3| reduces the force exerted b the spring when the casing on which the centering device is mounted is shifted closer to the wall of the confining hole, while providing an adequate centering or restoring force when the casing is disposed substantially greater distances off the wall of the hole.

The advantage of the intermediate flat spring portions is further emphasized from a consideration of Fig. 7, which discloses a comparison between three different forms of outwardly bowed springs. The centering device embodying spring R had each spring of ogive form, as shown in Fig. 5, the center bowed portion 39a having a six-inch rise in a thirty-six inch bowed length. The radius of curvature of the central convex portion 30a was 14 inches, whereas the radius of curvature of the end concave portions 3la was also 14 inches. The convex and concave portions 38a, 31a met at a common tangent or flex point 50.

Each centralizer spring T, shown in Fig. 4, also had the same thirty-six inch bowed length as spring R, the central bowed portion 30 having a six-inch rise. The radius of the convex central portion 30 was 10 inches, the radius of each concave end portion 3| was 10% inches, and the intermediate fiat portion 33 interconnecting the central portion 30 and each end concave portion 3! was 9 inches. The springs T of the centering device were made of the same spring steel as the centralizer with springs R, had the same cross-sectional area, were subjected to the same heat treatment, and were the same in number and circumferential disposition around the centering device.

The centering devices embodying springs R and T were designed to be mounted upon seveninch 0. D. casing.

In Fig. 7, the restoring force is plotted against the distance the casting is held 011 the wall of the hole. A centralizer with the ogive curved springs R exerts a progressively increasing restoring force in 17 inch 1. D. hole as the casing is moved laterally toward the wall of the hole. Thus, the force increases from about 700 lbs. when the casing is 4 inches ofi the wall or" the hole to about 2200 lbs. when the casing has been moved laterally until it is only /2 inches oil the wall of the hole. From that point on, as the casing is moved closer to the wall of the hole, the restoring force begins rising at an accelerated rate, and somewhat precipitously, being about "3400 lbs. when the casing is only inch on the wall of the hole. The rise is still steeper when the casing is moved still closer to the wall of the hole, as is clearly evident from curve R on Fig. 7

The ogive curved springs R may be compared with the restoring force exerted by the springs T in a centralizer having the intermediate fiat spring portions 33 of 9 inches in length between the central and end curved portions 30, M, with a radius of curvature of 10 inches. When the casing is held the distance of four inches off the wall of the 17 inches 1. D. hole, the restoring force for spring T is about 450 lbs., which is sufficient to insure the lateral displacement of the casing substantially oiT the wall of the hole. As the casing is moved laterally closer to the wall of the hole, it is found that the restoring force is about 1750 lbs. when the casing-is 1 inches or? the wall of the hole, which is less than the restoring force in curve R, but still very substantial, although not excessive. As the casing is moved still closer to the wall of the hole, the precipitous rise that occurred in connection with springs R does not occur in connection with springs T. Curve T on Fig. 7 discloses that a flattening out tendency occurs; so that the increase in the restoring force is at a decelerated rate until the casing is about -inch ofi the wall 'of the hole, acceleration of the restoring force not occurring until the casing is about -inch off the wall of the hole. At a distrnce of A -inch off the wall of the hole of the casing, the restoring force is about 2500 lbs. compared with about 3400 lbs. of the ogive curved spring R. Even at such great inward compression of a spring from a bowed height of six inches to a bowed height of only one-half inch, the spring T with the intermediate substantially flat portion is exerting 900 less pounds force than the centering device with springs R, or the latter springs are exerting about 35% more restoring force.

When the centering devices R and T are run in an 11 I. D. hole, the ogive curved springs R have the restoring force increasing greatly as the casing is moved closer to the wall of the hole, as is evident from the curve R shown in Fig. 7. The centering device with the springs T, however, heve the restoring force increasing at a decelerated rate over a large portion of the lateral movement of the casing toward the wall of the hole, as is evident from curve T in Fig. '7. As a matter of fact, there is actually substantially no increase in the restoring force in moving the casing from a distance of e g-inch to a distance of -inch from the wall of the 11 I. D. hole. From that point on, curve T rises precipitously, but, as a practical matter, the casing is not moved any closer to the wall of the hole than about /2inch, since the springs would bind upon the coupling collar D, or other stop devices, or upon other portions of the centering device. In any event, the restoring force of the springs T, as the casing is moved closer to the wall of the hole, increases to a much lesser extent than the restoring force exerted by the spring R.

springs R and T may be compared with spr n Y, which initially only has a 2-inch bowed spring rise, but which is also of ogive form. The precipitous rise of the restoring force is quite evident irom curve Y as the casing is moved closer to the wall of the hole. When run through close clearance surface casing, a centering device with springs Y, and with the 2-inch rise would have 10 a greater tendency to take a permanent deformation or set.

Fig. 8 discloses the starting and sliding forces required to move centering devices into and. within surface casing having different diametral clearances with respect to the casing on which each centering device is mounted. In this figure, the force exerted per spring is plotted against the diametral clearance between the casing string and the inner wall of the surface casing. Curve R plots the data obtained from cgive spring R referred to above. Curve T plots the data obtained from the tests on spring T referred to above, and curve 1 plots the data obtained from spring Y having the 2-inch bowed height and the ogive curve referred to above.

As the diametral clearance decreases, that is, as the centering device is run in closer clearance surface casing, the ogive curved springs R require a greater starting force per spring to start the centering device in the casing than the spring T, with the intermediate flat spring portions 33. The starting force required for the centering device with spring Y would be expected to be much less for larger diametral ciearances, since the initial bowed height is only two inches. However, curve Y, designating the starting force per spring, is rising precipitously, and would be greater than curve T for diarnetral clearances of less than 1% inches, which-would be a, clearance or" only A inch on each side of the casing. Inthe usual situation, however, the centering device would not be run into surface casing having a diametral clearance of less than about 1 /2 inches.

Curves R, Y and T can also be compared in connection with the force required to slide the centering device down through the surface casing. Curve Y rises quite precipitously as the diametral clearance decreases, the sliding force being rather substantial with smaller diametral clearances. Curve R, which is the ogive curved spring, also begins rising precipitously from a diametral clearance of about 2% inches, whereas curve T, for the spring including the intermediate fiat portions 33, does not begin rising precipitously until the diametral clearance is less than 1%; inches. At this latter clearance, the springs T, with the intermediate fiat portion 33, are exerting a substantially lesser sliding frictional force than springs R.

From the foregoing actual experimental data, it is evident that the intermediate fiat portion 33, or substantially flat portion, in the spring causes the restoring force of the centralizer to increase at a much lesser rate than the restoring force exerted by other forms of springs, such as the ogive spring that does not have the inter mediate fiat portions. Thus, the springs with the intermediate fiat portion can be compressed closer to the casing on which the centering device is mounted without fear of the springs taleing a permanent set. The force exerted by these springs, when they occupy a large bcwed height from the casing, is more than sufiicient to hold the casing oil the wall of the hole; so that the centering device is efiective to center the casing in holes that are much greater in diameter thanthe casing on which the centering device is mounted, as well as in holes that have cavities in which the centering device might be disposed when the casing is landed in in the hole. The fact that the springs with the flattened portions 33 exert a much lesser restoring force makes it easier to start centering devices embodying such springs in close clearance surface casing, without the necessity for the application of forces other than those exerted by the weight of the casing itself. Not only can the centering device be started in close clearance surface casing with facility, but there is much less sliding friction between the springs and the wall of the surface casing, minimizing the extent of wear on the springs and insuring that they are capable of efiectively exerting adequate centering force when disposed in the hole.

The inventor claims:

1. In centering devices adapted to be mounted on well conduits: longitudinally spaced supporting members; preformed circumferentially spaced, outwardly bowed springs extending between and carried by said supporting members; each spring including a central outwardly convex curved portion and outwardly concave end portions; each spring also having an intermediate straight portion of extended length extending between said central portion and each end portion.

2. In centering devices adapted to be mounted on well conduits: longitudinally spaced supporting members; circumferentially spaced, outwardly bowed springs extending between and carried by said supporting members; each spring including a central portion embodying an out wardly convex curved region, each spring having end outwardly concave portions merging into terminal portions substantially parallel to the axis of the supporting members, each spring having an inclined intermediate substantially straight portion of extended length extending between said central portion and each end portion, each of said intermediate portions being tangent to said central portion and end portion.

3. In centering devices adapted to be mounted on well conduits: longitudinally spaced supporting members; circumferentially spaced, outwardly bowed springs extending between and carried by said supporting members; each spring including a central portion embodying an outwardly convex curved region, each spring having end outwardly concave portions, each spring having an inclined intermediate substantially straight portion of extended length extending between said central portion and each end portion, each of said intermediate portions being tangent to said central portion and end portion.

WILLIAM S. ALTHOUSE, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,685,380 Shultz Sept. 25, 1928 1,773,398 'La Velle Aug. 19, 1930 1,820,391 Hartman Aug. 25, 1931 2,174,085 Hartman Sept. 26, 1939 2,424,027 Gist July 15, 1947 2,482,985 Lockwood Sept. 27, 1949 2,546,582 Baker Mar. 27, 1951 

